Many fluid flow control devices, such as check valves, are spring or magnetically biased towards an open position. If a differential pressure exceeds a predetermined value, the valve is biased closed.
However, there are certain disadvantages in using spring biased devices like spring biased poppet-type check valves. The spring exerts increasing force against the flow as the poppet is closing and, thus, closing action is less positive and less reproducible.
Further, poppet valves and associated springs are relatively heavy, are adversely affected by gravity, and may be indifferently oriented relative to the direction of gravity forces, thereby making low flow pressure operating conditions difficult for proper operation.
Similarly, magnets that hold check valves may be required to be larger under certain operating conditions and may minimize the effective flow capacity of fluid flowing through a pipe.